The operation of internal combustion engines, regardless of the particular internal combustion engine design, such as reciprocating engines, piston-less rotary engines or free-piston engines, occurs in repeated strokes in each of which certain processes are carried out, such as intake and compression of a fuel and air mixture, combustion, and discharging of the combusted fuel air mixture, or the like. The sounds generated hereby partially propagate through the engine directly as solid-borne sound. Another portion of the sound generated exits along with combustion gases through an exhaust system of the engine as air-borne sound. In an exhaust line of the exhaust system, this air-born sound is superimposed by flow noise of the combustion gases. The sound resulting from this superimposition is called exhaust sound. Finally, a remaining part of the sound generated exits through an intake system of the engine.
The sounds propagated through the internal combustion engine as solid-borne sound can generally be well insulated by suitable insulating materials in the engine compartment of a vehicle.
To reduce the acoustic emissions escaping with the exhaust gases, sound-absorbing devices are usually arranged in the exhaust duct. Such sound-absorbing devices are called mufflers. Mufflers can operate, for example, according to the absorption and/or reflection principle. Furthermore, it is known to provide mufflers with resonating chambers harmonically tuned to cause destructive interference wherein opposite sound waves cancel each other out.
It is a disadvantage with such systems that they increase the back pressure of the exhaust gas flowing in the exhaust gas system, thus decreasing efficiency of the combustion engine. It is a further disadvantage with such systems that, especially in the case of modern diesel vehicles, and vehicles with hybrid drive systems, the sound actually leaving the exhaust gas system is not appealing to a user.
Therefore, active sound systems have been developed for use in exhaust systems of vehicles with which it is possible to generate an exhaust sound synthetically. Corresponding systems have an electro-acoustical transducer that is connected to the exhaust line of an internal combustion engine by a connector piece in order to superimpose electro-acoustically generated sound waves on the sound waves stemming from the combustion process in the engine or generated by the flow of exhaust gas in the exhaust gas system. In this way, the exhaust sounds of a vehicle can be deliberately modified. The electrical input signal of the transducer is generated by a control as a so-called control signal, taking into account current values of engine parameters, such as engine speed or firing order. The electro-acoustical transducer is housed by a housing separate from the exhaust line and thus requires additional space in the undercarriage of a vehicle.
Active sound systems may be used as anti-noise systems as well as an alternative or supplement to mufflers, for example. Anti-noise systems superimpose electro-acoustically generated anti-noise on airborne noise generated by the internal combustion engine and propagated through the exhaust system. Respective anti-noise systems may use a so-called Filtered-X, Least Mean Squares (FxLMS) algorithm trying to bring the airborne noise propagating through the exhaust system down to zero (in the case of noise-cancellation) or to a preset threshold (in the case of influencing noise) by outputting sound using at least one loudspeaker. The loudspeaker of anti-noise systems is usually in fluid communication with the exhaust system. For achieving a completely destructive interference between the sound waves of the airborne sound propagating through the exhaust system and the anti-noise generated by the loudspeaker, the sound waves originating from the loudspeaker have to match the sound waves propagating through the exhaust system in amplitude and frequency with a relative phase shift of 180 degrees. If the sound waves of the airborne noise propagating through the exhaust system match the anti-noise sound waves generated at the loudspeaker in frequency and have a phase shift of 180 degrees relative thereto, but do not match in amplitude, only an attenuation of the sound waves of the airborne sound propagating through the exhaust system results. The anti-noise may be calculated separately for each frequency band of the airborne noise propagating through the exhaust pipe using the FxLMS-algorithm by determining a proper frequency and phasing of two sine oscillations being shifted with respect to each other by 90 degrees, and by calculating the required amplitudes for these sine oscillations. Respective systems are for instance known from the following documents: U.S. Pat. Nos. 4,177,874, 5,229,556, 5,233,137, 5,343,533, 5,336,856, 5,432,857, 5,600,106, 5,619,020, EP 0 373 188, EP 0 674 097, EP 0 755 045, EP 0 916 817, EP 1 055 804, EP 1 627 996, DE 197 51 596, DE 10 2006 042 224, DE 10 2008 018 085 and DE 10 2009 031 848.
The objective of active sound systems may be that the cancellation or influencing of sound is audible and measurable at least outside of the exhaust system. As the case may be, the cancellation or influencing of sound is audible and measurable also inside the exhaust system.
It is a disadvantage with such active sound systems that they need to be fail-safe to meet legal provisions of noise protection. Therefore, they are often used as a supplement to existing mufflers. However, the space in the undercarriage of a vehicle is very limited.